1. Field of the Invention
The present invention relates to the field of shared multipoint-to-multipoint communication networks.
The present invention will be described hereafter in relation with an example of application to networks using the electric supply conductors (for example, the mains) as a transmission medium. High-frequency sub-carriers which are modulated to transmit data between one or several devices equipped with modems (modulator-demodulator) and connected to the mains are generally used. Such networks may be used, for example, to connect a microcomputer to its peripherals (printer, scanner, etc.). They may also distribute, within a home, an office or the like, multimedia data coming from a connection to an external access, for example, a satellite antenna, an optical fiber cable, a modem cable, an XDSL modem, etc.
2. Discussion of the Related Art
Since various devices may simultaneously need to transmit information over the shared network, an access control mechanism of the transmission means (MAC) is necessary to avoid collisions which would result in information losses. Further, since different multimedia services or transmission types are likely to use the same electric supply conductors and since these different transmissions most often have distinct constraints in terms of delay, bit error rate, etc., an access priority management mechanism more generally designated as a quality-of-service control (QOS) is generally used.
FIG. 1 very schematically shows an exemplary architecture of a transmission network using electric supply conductors L as a transmission medium. The electric supply network connects different taps P together, possibly via an electric board provided with circuit breakers or the like (not shown). Taps P have been symbolized in FIG. 1 as being taps with three conductors (phase, neutral and ground). However, these also may be taps only having two conductors (phase and neutral). From the point of view of the transmission network, each tap is considered as a node N1, . . . , Ni, . . . , Nj, . . . , Nn.
Among the various electric devices connected to the network, devices 1 of a first type are equipped with a modem Mi, Mj respectively connected to nodes Ni, Nj to communicate over the network thus formed. In the example of FIG. 1, a device 2 connected on one of the taps is a modem-free device which is only supplied by the electric network.
Most often, each device 1 of the first type connected to any tap from the point of view of the electric supply (node from the point of view of the network) should be able to send and receive data. The network is accordingly a so-called point-to-multipoint or multipoint-to-multipoint network.
The data transmission protocols over shared networks can be grouped in three large categories. A first category concerns time-division multiple accesses (TDMA or TDD), which assign different time slots to each transmission. A second category groups code-division multiple accesses (CDMA) which assign different codes for each transmission. A third category concerns frequency-division multiple accesses (FDMA or FDD), which assign one or several frequencies to each transmission.
All these transmission systems are generally used in multipoint to point systems such as, for example, GSM mobile telephony communication systems. They however all have disadvantages in the point-to-multipoint or multipoint-to-multipoint communication systems to which the present invention applies.
For time-division transmissions, significant dead times must be provided between each data sequence sent by each device. These dead times must be inserted to avoid collisions between packets transmitted by different nodes, while taking account of all possible reflections and multiple paths between the transmitter and the receiver. This disadvantage can significantly reduce the network capacity due to the decrease in general transmission rate.
For code-division transmissions, flow rate limitations on the order of a few hundreds of kilobits per second (at most, a few megabits per second) are observed in practice due to the complexity of managing the multiple users which causes a lot of interference, and to the level differences of the received signal according to the paths to be followed by the different signals. The implementation of an efficient code-division system requires significant means, which can quickly reach a prohibitive cost. Further, the spectral density of a code-division transmission extends over the entire usable bandwidth, which makes this type of transmission incompatible with electromagnetic compatibility requirements, which require being able to forbid transmissions in certain specific frequency bands.
The present invention more specifically relates to a frequency-division transmission which, with current techniques, remains poorly adapted to point-to-multipoint or multipoint-to-multipoint transmissions in a network having more than two nodes.
Indeed, in a frequency-division multiple access, each device is assigned a predefined set of frequencies to transmit data. On the receive side, there exist two solutions. Either a communication control channel is used to indicate which frequencies must be received and demodulated by each device connected to the multipoint-to-multipoint network. Or all receive frequencies are demodulated by all devices and each receive device selects the information intended for it.
A disadvantage of the conventional frequency-division multiple access is that it requires analog filters to separate the frequencies or groups of frequencies used for the modem transmit and receive sections. This is a major disadvantage in terms of flexibility and frequency adaptation capability since the analog filters which are formed cannot be modified according to the dynamic capacitance needs or to any other reason requiring modification of the assigned frequencies. On this regard, a specificity of networks using the supply conductors as a transmission support is that the network transfer function is likely to strongly vary, for example, upon plugging of an electric device (be it or not equipped with a modem) on the mains.
Conventionally, standards relative to networks using the supply network as a transmission medium provide the combined use of a frequency-division multiple access and of a time-division multiple access. This actually is a carrier detection and collision or anticollision detection multiple access (CSMA/CA). According to these transmission standards, the frequency-division multiple access is implemented by using an orthogonal frequency-division multiplexing (OFDM).
This is a well known technique which will be briefly reminded hereafter. Reference can also be made to literature. For example, article “HomePlug Standard Brings Networking to the Home” by Steve Gardner, Brian Markwalter, and Larry Yonge, published in December 2000 in Communication Systems Design, which is incorporated herein by reference, discusses the application of such a multiplexing to networks using electric power cables as a transmission support.
The OFDM waveforms are generated by using inverse Fourier transforms (IFFT) in which the points of the frequency field are formed by complex sets of symbols which modulate each carrier. The result of the inverse Fourier transform is called an OFDM symbol. On the receive side, the data are reconstituted from a direct Fourier transform which converts the OFDM symbol in the frequency field.
The present invention aims at providing a novel data transmission technique by orthogonal frequency-division multiplexing which avoids use of analog filters to separate the frequencies or groups of frequencies used by modems.
The present invention takes its inspiration from a transmission technique known in telephony which is known for being more flexible in terms of frequency assignment. This technique, known as the Zipper-DMT, is only conventionally used for point-to-point transmissions and is described, for example, in European patent application No. 0,883,944 and in article “Zipper VDSL: A Solution for Robust Duplex Communication over Telephone Lines” by Denis J. G. Mestdagh, Michael R. Isaksson, and Per Ödling, published in May 2000 in IEEE Communication Magazine, pages 90 to 96, which is incorporated herein by reference.
In the application to point-to-point communication systems, different frequency bands are assigned for the transmission and reception (rising direction, falling direction). In this known application to telephony, the assigned frequency bands are the same for all the wires of a same cable and, to avoid a crosstalk phenomenon, the frequencies are not assigned to the different modems but to different cables.
This promising technique is however not directly transposable to networks using the electric supply network as a transmission support.
Among the additional problems encountered in this type of network, which make the application of the conventional Zipper-DMT technique poorly adapted, the fact that the transfer function of the transmission support considerably varies along time (for example, under the effect of the branching of an electric device), that the transfer function varies from one node to the other, and the fact that it is a multiple-path and multiple-reflection network, should be noted.
WO-A-99 23764 discloses a data transmission system by frequency-division multiplexing using cyclic prefixes.
EP-A-1 065 818 discloses a transmission system wherein a distinct frequency set can be allocated to each network node.
GB-A-2 332 602 discloses a multidirectional transmission system providing sync messages at the end of the frames. These messages aim at transmission synchronization.